I think you're better off spending your time and money elsewhere; you're not going to see anything spectacular

I went to Liechtenstein on a day trip from Zurich; there's nothing to see in Liechtenstein really, I wouldn't be bothered visiting it's just a basic little town with a mediocre castle. I honestly just went for the sake of saying I've been to Liechtenstein

I decided to have a 4 day holiday in Luxembourg City. I seen pretty much the entire city in 6 hours. The next day I went to Vianden Castle which was nice. The other two days I had nothing to do so I just went on day trips to Trier, Germany and Metz, France. I really enjoyed Trier it was probably the highlight of my trip.

Both countries are expensive; unless you're rich as fuck and have lots of time to kill I wouldn't really bother

They should probably be near the bottom of your list of European countries to visit desu.

In Luxemburg you can use the entire public transportation system for a whole day for 4 euros.
Maybe consider Echternach.